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Adjusted Stats 2009 Year in Review Part VII

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This is the seventh in a series of posts that will review the 2009 season (as well as 2008) from an adjusted stats perspective.

Tentative Publication Schedule

Part I: Which Stats Correlate Best to Winning? - 6/24/10
Part II: Drilling Down and Regression Models - 6/28/10
Part III: Passing Efficiency Formula - 6/30/10
Part IV: Conference Strengths and Pace - 7/2/10
Part V: Testing Conventional Wisdom - 7/16/10
Part VI: Team Matchups - 7/20/10
Part VII: Year-to-Year Changes - 7/22/10
Part VIII: Points per Yard Efficiencies - 7/26/10
Part IX: Data Dump - Team Rankings - 7/28/10
Part X: Data Dump - Team Reports - 7/30/10

Year-to-Year Changes

The team reports that will be dumped on you next week include data from both 2008 and 2009 along with a column detailing the year-to-year change for each team in each statistic. With that information we can see how a team improved or got worse at any phase of the game for which we have data. Some statistics we know are due almost entirely to skill and execution, such as total rushing per carry. But some statistics seem to have a lot to do with luck, such as percentage of forced fumbles recovered. There are some seasons where a team seems to recover far more than their share of fumbles either offensively or defensively. The percentage of fumbles recovered by the defense should always end up near 50% (it was 50.2% nationally in 2009 and 49.1% in 2008), something I’m confident of to the point that I don’t run an adjusted stat for this.

But should I? Although I don’t have an adjusted stat available to run the study, the raw percentage of forced fumbles recovered stat should serve us just fine here. If a defense truly swarms to the ball more than the average team to the point that we should expect them to consistently recover more fumbles than other teams, that should show up in the raw stat. A coefficient of 0.116 exists for correlation between recovered fumble percentage in 2008 and the same in 2009. This is a positive correlation but falls below the 0.200 statistical significance threshold that we’ve been using. It is a higher value, though, than the 0.053 coefficient for adjusted forced fumbles per carry from 2008 to 2009. Clearly a team’s performance when it comes to fumbles can’t be expected to remain consistent from year-to-year. Here is a table showing both offensive and defensive fumble-related correlations from 2008 to 2009:

Statistic Coefficient
Adjusted Fumbles per Carry 0.266
Non-Adjusted Fumbles per Carry 0.264
Offensive Fumble Recovery Percentage (non-adjusted) 0.178
Defensive Fumble Recovery Percentage (non-adjusted) 0.116
Adjusted Forced Fumbles per Carry 0.053
Non-Adjusted Forced Fumbles per Carry -0.013

It appears that the coachspeak about emphasizing ball protection for a team’s offense is well-founded. The strongest correlation is for the frequency with which a team fumbles the ball. On the other hand, coachspeak about emphasizing stripping the ball and forcing fumbles appears to be largely without merit. The frequency with which a defense forces fumbles is statistically non-correlated from one year to the next. Fumble recovery percentage shows some possible correlation but not high enough to be certain; perhaps the correlation that does exist is linked to run/pass ratio for an offense. Fumbles after receptions are more frequently recovered by the defense, but fumbles after sacks are more frequently recovered by the offense (note: those are assumptions), so it may be related to pass protection in combination with playcalling ratio. Regardless, looking at the defensive statistics above it’s clear that you should not expect a team that performs either extremely well or extremely poorly in forcing and recovering fumbles in one season to repeat that performance the next year.

Before proceeding to cover all the year-to-year correlations for tracked statistics let’s take a look at the rest of the turnover-related statistics:

Statistic Coefficient
Adjusted Turnovers Forced per Possession 0.274
Adjusted Interceptions per Attempt 0.231
Turnovers Forced per Possession 0.212
Interceptions per Attempt 0.170
Adjusted Turnovers Lost per Possession 0.099
Turnovers Lost per Possession 0.046
Interceptions Thrown per Attempt 0.022
Adjusted Interceptions Thrown per Attempt 0.006

What to make of these results? In the fumble section above, I gave credit to coaches and teams that emphasize protecting the ball. But here we see that throwing interceptions shows basically zero correlation from one year to the next. It could be a case of quarterbacks being replaced if they throw too many interceptions and that interceptions are more directly related to individual execution than fumbles, which may be somewhat chaotic or random. What we do see, though, is that while defenses are not consistent in their year-to-year ability to cause or recover fumbles they do show more consistency when it comes to intercepting opposition pass attempts.

So there’s a lot to consider when we’re looking at year-to-year correlations. If a statistic can be easily influenced by a single player then we will likely see lower correlation coefficients. If there’s a significant randomness factor contributing to the statistic we will also see weaker correlations. And with the significant annual roster turnover in college sports how large can we truly expect any coefficient to be? The next page shows a table detailing the year-to-year coefficients for all other available adjusted stats. The first two tables showed a decent enough level of agreement between the raw and adjusted correlations (with the adjusted numbers appropriately showing greater correlation in most cases) that the adjusted will be used moving forward.

What should we expect? Based on the above logic we would expect stats obtained by the full team to show the most correlation as roster turnover should affect the entire team less drastically than it may affect individual positions. Taking the next logical step we would anticipate that full unit statistics, i.e., overall offensive and defensive statistics, to show stronger correlation than more specific stats. And as you’ll see while reviewing the table that logic holds fairly steady. But there’s something else made obvious when looking at the results that I hadn’t considered before running the numbers. Take a look at the table and come back.

The clear trend is that defensive performance correlates better from year-to-year than offensive performance. What are the possible explanations and associated strategic implications? Above I posited that individual focus can decrease year-to-year correlation. And the defensive side of the ball is less prone to being dominated by individual players than the offensive side. On offense, one player at any time possesses the ball and that player will have a greater impact on his unit’s performance than his teammates.

A second possible explanation is that offenses are more prone to change overall strategy and tactics on a yearly basis than defenses. The variety of defensive strategy is lower than offensive, which should cause greater stability on the defensive side of the ball and therefore stronger year-to-year correlations. Anecdotally it seems that the game of cat-and-mouse is usually led by offensive innovation from a few teams and then others will follow while the strategic defensive responses spread sooner throughout the country if they show success against the latest offensive attacks.

So now here's the really long table showing the year-to-year correlation results for the rest of the adjusted stats in my database:

Rank Statistic Coefficient
1 Point Margin per Game 0.784
2 Points Allowed per Game 0.780
3 Point Margin per Play 0.776
4 Point Margin per Possession 0.774
5 Points Allowed per Possession 0.764
6 Points Allowed per Play 0.760
7 Winning Percentage 0.734
8 Rushing Yards Allowed per Game 0.698
9 Yards Allowed per Play 0.691
10 Total Rushing Allowed per Game 0.687
11 Yards Allowed per Game 0.685
12 Yards Allowed per Possession 0.680
13 Yards Allowed per Carry 0.658
14 Total Passing Allowed per Att. 0.646
15 Passing Efficiency Allowed 0.632
16 Total Rushing per Game 0.611
17 Rushing Yards per Game 0.611
18 Total Rushing Allowed per Carry 0.598
19 Yards Allowed per Att. 0.596
20 Plays per Game 0.586
21 Points per Game 0.584
22 Points per Play 0.567
23 Passing Yards per Game 0.556
24 Total Passing per Game 0.552
25 Points per Possession 0.546
26 Penalty Yards per Game 0.537
27 Sacks per Game 0.531
28 Yards per Game 0.494
29 Penalties per Game 0.477
30 Yards per Carry 0.466
31 Yards per Play 0.462
32 Total Rushing per Carry 0.449
33 Yards per Possession 0.446
34 Sacks per Att. 0.445
35 Total Passing Allowed per Game 0.445
36 Sacks Allowed per Att. 0.425
37 Passing Yards Allowed per Game 0.409
38 Total Passing per Att. 0.408
39 Yards per Att. 0.391
40 Interceptions per Game 0.376
41 Sacks Allowed per Game 0.375
42 Possessions per Game 0.361
43 Passing Efficiency 0.355
44 Touchback Percentage (non-adj.) 0.340
45 Time of Possession 0.337
46 Net Kickoff Average 0.329
47 Turnovers Forced per Play 0.320
48 Turnovers Forced per Game 0.301
49 Field Goals per Game 0.280
50 4th Down Conversion % Allowed 0.276
51 3rd Down Conversion % 0.265
52 Punt Return Average 0.263
53 Fumbles per Game 0.239
54 Turnover Margin per Possession 0.237
55 Time per Offensive Play 0.236
56 Red Zone Scoring % 0.170
57 Field Goal % 0.168
58 Kickoff Return Average 0.167
59 4th Down Conversion % 0.165
60 Net Punting Average 0.153
61 Turnover Margin per Play 0.148
62 Turnover Margin per Game 0.147
63 3rd Down Conversion % Allowed 0.140
64 Interceptions Thrown per Game 0.139
65 Turnovers Lost per Play 0.132
66 Turnovers Lost per Game 0.108
67 Offensive Plays per Game 0.107
68 Defensive Plays per Game 0.065
69 Red Zone Scoring % Allowed 0.064
70 Forced Fumbles per Game -0.024

Given that defensive performance is more consistent from season to season, it would seem that identifying and keeping a solid defensive coordinator and staff would be extremely important to a program. The program can be confident that the defensive performance is more likely to stay above par on an annual basis and that any difficulties on the offensive side of the ball can be addressed more quickly via personnel or strategic changes. So should teams look for defensive-minded head coaches versus offensive-oriented leaders? That could go either way because if a school can find an offensive head coach that can deliver consistent performance on that side of the ball then they will have a huge leg up on the competition. Thoughts?